form in window ,tab key can't switch field

I upgrade to extjs5,I had a form in one window,some fields in the form,when the modal of the window is true,I can't use tab key to switch fields,however when modal is false the tab key can use again,why? how to solute it?

my code like this:

Code:

Ext.create('Ext.window.Window', {
    title: 'Hello',
    modal:true,//this，you can set false to try。
    width: 400,
    items: [{  // Let's put an empty grid in just to illustrate fit layout
        xtype: 'form',
        border: false, 
        height: 200,
        items: [
                    {
                        allowBlank: false,
                        fieldLabel: 'User ID',
                        name: 'user',
                        xtype:'textfield',
                        emptyText: 'user id'
                    }, {
                        allowBlank: false,
                        fieldLabel: 'Password',
                        name: 'pass',
                        xtype:'textfield',
                        emptyText: 'password',
                        inputType: 'password'
                    }
        ]
    }]
}).show();

form in window ,tab key can't switch field